Problem: I have a list of pairs (md5_hash, id). I want to store the data in MySQL and I want to make queries like this
SELECT id FROM table WHERE md5_hash = <some_hash>
The number of pairs can be tens or hundred of millions and the number is static - i.e., I do not add new records. It is important for me to save disk space. The time is not so important in this case (if a lookup takes less than say 1 second it is okey).
I started by creating a table where md5_hash is represented in this way:
CREATE TABLE `myTable` ( `md5` binary(16) NOT NULL, ' the values are not unique but we can suppose they are. `id` int(10) unsigned NOT NULL ) ENGINE=MyISAM
so md5 hashes are represented as binary numbers instead of hexadecimal characters to save space. For the same reasons the columns are defined as "NOT NULL". I chose MyISAM engine because my tests show that MyISAM requires less disk space than InnoDB. In addition MyISAM tables can be compressed by myisampack utility.
Now the hard part comes. When I create an index on md5 column the index requires more disk space than the data itself! I tested it with 17 millions records and my table took around 300 MB of disk space and the index took about 330 MB of disk space. The size of the index is crazy.
One idea is to provide the md5 hashes presorted to MySQL which should result in smaller index size. But I do not know how to do that.
Another idea is to divide myTable in several smaller tables in order to decrease size of the index. I tried mysql partitioning. However, the purpose of the feature is to improve query time and not disk space usage.
Do you have any idea how to decrease the required disk space usage?